Step 1. For this project I am using a piece of sil nylon 26" by 21". This will make a stuff sack 8" in diameter by 16 inches deep. Fold it in half along the 26" length and run a simple seam to join the two edges with a 1/4" seam allowance. If you're using a coated material other than sil nylon, make sure the coated sides are out. Run another line of stitching along the bottom.

Forming the Bottom

Step 2: Pull apart the two sides of the bag. This allows you to get to the fabric to form the seams that make up the bottom of the stuff sack.

Step 3: Measure the bottom seam and divide the length by 4. Make a small mark along the bottom 1/4 of the way from each edge. In this example, the bottom length is 12" long, so I made a mark 3 inches from each side. You'll sew across the bottom, intersecting these marks, to form the bottom's side seams.

Step 4: Move the ruler perpendicular to the mark you just made and make two small guide marks on the fabric. You'll use these marks to help you make a straight line of stitching in the next step. Repeat on the other side

Step 5: Run a line of stitching connecting the three marks as shown in the picture. Secure the start and end of your line of stitching with a few reverse stitches. Repeat on the other side. These are the bottom of the stuff sack's side seams.

Step 6: Cut off the extra fabric about 1/2" from the line of stitching. Tie off the loose ends. Now the bottom is finished!

Making the drawcord Channel

Step 7: Remove the accessory tray of your sewing machine to gain access to the free arm. Run a 1/4 inch hem along the top of the stuff sack. The sack will still be inside out.

Step
8: Replace the accessory tray so that you can make a manual
buttonhole. I make the hole opposite from the main seam, but this isn't
absolutely neccessary. Reinforce the buttonhole with a scrap of fabric from
the material you cut off the bottom in step seven. This buttonhole protects
the fabric from the strain of pulling the drawcord. Another option is to
use a metal grommet.

Step 9: Use the seam ripper to open up the buttonhole. This is where the drawcord will exit the drawcord channel.

Step 10: Remove the accessory tray again to gain access to the free arm. Fold back the lip of the stuff sack and run another line of stitching to form the tube that the drawcord will go through. Secure the start and finish of your stitches with a few reverse stitches. I start and finish this line at the reinforcement patch at the buttonhole.

Step 11: Tie off and trim all the loose ends, then turn the bag rightside out. Feed a piece of insulated wire through the drawcord channel. Make sure that the end of the wire doens't have any sharp edges to avoid puncturing your bag.

Step 12. Secure your drawcord to the wire with a piece of tape and then pull the wire to feed the drawcord through the channel. I use flat polypropylene or nylon cord, but round, acccessory cord works well too. Finish it off with a cordlock if desired.

Here's the finished product. Stuff sacks like these sell for between $10 and $15 dollars.

Make sure you seal
the seams carefully if you want the stuff sack to be fully waterproof.
If you used sil-nylon, make sure you seal the seams with McNett's Sil
Net or other 100% silicone seam sealer. Support this site by buying our
kit that contains the fabric,
drawcord, and cord locks.

Replace your sewing machine needle with the new needle. It's always a good idea to start any project with a fresh needle. Set up the machine with your 100% polyester thread. Don't use any other type of thread. Poor thread ruins more gear projects than anything else. Check your machine's tension and adjust if neccessary.

AYCE says

Making your own gear is a lot of fun! This project is a great way to learn how to set up your machine and learn how to sew. Some outdoor fabrics can be a real challenge to work with. In particular, sil nylon is very slippery and requires some practice to get it to feed right. Pay very careful attention to the foot pressure of your machine, and definitely make sure your stitch is balanced. Above all, practice, practice, practice.